1. Field of the Invention
The present invention relates to a solid-state imaging device constituted by a semiconductor device, and more particularly to an XY address-type solid-state imaging device manufactured by a CMOS process. In addition, the present invention relates to a solid-state imaging device capable of eliminating the effects of fixed pattern noise and thermal noise.
2. Description of the Related Art
Solid-state imaging devices include XY address-type solid-state imaging devices in which an image sensor is formed by CMOS, and the so-called CCD solid-state imaging devices constituting a charge transfer-type image sensor. Because the XY address-type solid-state imaging device, which utilizes a CMOS image sensor, does not require a special manufacturing process, and is driven by a single power source with smaller power consumption, and furthermore, is capable of mounting various signal processing circuits on the same chip, it is seen as a promising replacement for CCD solid-state imaging devices.
This CMOS image sensor-equipped conventional XY address-type solid-state imaging device will be explained by using FIG. 7. FIG. 7 shows an example circuit worth of one picture element (pixel) of a conventional XY address-type image sensor. The conventional CMOS image sensor shown in FIG. 7 has a constitution called an APS (Active Pixel Sensor), which mounts a source follower or other such buffer 404 to each pixel. The cathode side of a photodiode 400 is connected to the gate electrode of the buffer 404, and to an MOSFET reset switch 402. Further, buffer 404 is connected to a vertical selection line 408 via a horizontal selection switch 406.
The operation of this conventional XY address-type solid-state imaging device will be briefly explained. First, when reset switch 402 is turned ON at a prescribed timing by a reset signal RST, photodiode 400 is charged to a reset potential VRST. Next, the discharge of photodiode 400 commences in line with an incoming light, and the potential decreases from the reset potential VRST. After the passage of a prescribed time, when a horizontal selection signal RWn is input to the gate electrode of horizontal selection switch 406, and horizontal selection switch 406 transitions ON, a source voltage of buffer 404 is extracted as a signal voltage via vertical selection line 408.
However, a conventional APS of the above constitution has a charge storage capacitor and a source follower or other such amplifier, a fixed pattern noise (FPN) in generated. This noise is DC output level fluctuation for the same signal according to VT (threshold voltage) variation of the source follower transistor. It results in picture quality deterioration. That is, the detection voltage varies between cells for the same quantity of light received, resulting from built-in variations in the threshold voltage VT of the source follower transistor 404.
To reduce this variation, in the conventional device, after sampling an integral level conforming to a quantity of received light as a source signal voltage V1 of buffer 404, photodiode 400 is reset to the reset potential VRST, and this reset voltage is sampled. Then, fixed pattern noise is reduced by determining, using a correlated double sampling (CDS) circuit, the voltage difference between a source signal voltage V2 corresponding to reset voltage VRST and the above-mentioned source signal voltage V1. In other words, by sampling a reset voltage after storing a quantity-of-light signal, and determining, via a correlated double sampling circuit (CDS circuit), the difference with the signal voltage at the time of storing the quantity-of-light signal, the effects of threshold voltage VT variations are removed, and the fixed pattern noise (FPN) is reduced.
However, with this method, there remains the problem that the reset noise (kTC noise) before storing a quantity-of-light signal and the reset noise after reading the signal are added so as to increase the random noise level, and the S/N ratio deteriorates compared to that of a CCD solid-state imaging device.
kTC noise (where k is Boltzmann's constant, T is absolute temperature, and C is the capacitor of photodiode 400) is a kind of thermal noise. When reset switch 402 is made conductive by a reset signal RST, and photodiode 400 charges to a reset voltage VRST, the cathode terminal voltage of this parasitic capacitance is subject to the fluctuations of the thermal noise 4kTRΔf (where R is the resistance of reset switch 402, and Δf is the frequency range at charge time) from the reset voltage VRST. As a result thereof, the cathode terminal voltage resulting from a reset operation is not necessarily becoming a constant reset voltage VRST.
The above-mentioned conventional example uses the difference between a quantity-of-light signal level whose voltage drops from an initial reset level conforming to a quantity of received light, and a reset level of immediately thereafter. However, since this kTC noise has random fluctuations with time as hereinabove, the kTC noise that is superimposed on the initial reset level differs from the kTC noise that is superimposed on the second reset level, making it impossible to curb kTC noise even by using the difference between these two levels to curb fixed pattern noise (variations in threshold voltage VT).
Next, an XY address-type solid-state imaging device disclosed in Japanese Patent Application Laid-open No. 8-205034 will be explained by using FIG. 8. In FIG. 8, a source follower-type buffer B1 is connected between a frame transfer gate FT and an MOS-type switch SY1. Further, an MOSFET-constituted reset switch SR1 is connected to a second capacitor C2 for removing a charge that is stored in the second capacitor C2. The drain electrode of a buffer B1 is connected to a power source VDD, and the source electrode of the buffer B1 is connected to a horizontal selection switch SY1. Further, the gate electrode of buffer B1 is connected to second capacitor C2. Reset potential VR is applied to the drain electrode of reset switch SR1. The source electrode of reset switch SR1 is connected to the second capacitor C2, and the gate electrode of reset switch SR1 is connected to reset control signal line L2.
When a charge is transferred to the second capacitor C2 by making frame transfer gate FT conductive after a charge has been stored in a first capacitor C1, the gate potential of the buffer B1 steadily increases. When horizontal selection switch SY1 transitions ON after the passage of a prescribed time, the source voltage of buffer B1 is output via a vertical selection line, and a quantity of charge Q stored in the second capacitor C2 can be detected. By making reset switch SR1 conductive one time prior to making the frame transfer gate FT conductive, all of the charge stored in the second capacitor C2 can be removed, making it possible to curb the deterioration of picture quality resulting from a residual image charge.
However, for the conventional example of FIG. 8, firstly, the problem is that because there is a buffer B1, which is a source follower, it is not possible to remove the effects of threshold voltage variations (fixed pattern noise) in the transistor B1, and the detection level between cells varies. Furthermore, in Japanese Patent Application Laid-open No. 8-205034, there is no disclosure at all that offers suggestions concerning curbing this fixed pattern noise. And it is also not possible to remove kTC noise, which is thermal noise generated at reset, because after the second capacitor C2 is charged to reset voltage VR, a charge that accords with a quantity of light is transferred via gate FT from the first capacitor C1, a voltage that accords with the quantity of light is lowered from reset voltage VR, and only this voltage is transferred to a charge readout portion.
Further, as for the device constitution of the pixel shown in FIG. 8, the problem is that there is at the least 1 more transistor than the device constitution of the pixel shown in FIG. 7 (2 if reset switch 402 shown in FIG. 7 is added), constitution of the pixel portion is complex, and the numerical aperture of the light reception portion (fill factor) decreases.